Investigating the Effect of Storage Time on the Mechanical Properties of Biodegradable Polylactic Acid Film Containing Zinc Oxide Nanoparticles

Introduction Polylactic acid (PLA) is a biodegradable polymer that can replace petroleum-based materials in packaging films due to its unique properties. However, sometimes the degradability of polymers can be considered a negative factor, such as when significant changes in the mechanical properties of the polymer occur during use. Another notable issue is the brittleness of polylactic acid, which can be modified to some extent by adding other materials. The addition of materials such as nanoparticles and plasticizers can improve the flexibility and mechanical properties of polymer films. Polymer films must possess acceptable physical, mechanical, thermal, and other relevant characteristics for use in the packaging industry. The acceptable level of these properties can be obtained by comparing them with the established standards for commonly used polymers in the industry. Low density polyethylene (LDPE) is a polymer widely used in the packaging industry, making it a good benchmark for comparison. This research focused on studying various factors affecting the quality of the produced films, including mechanical properties, light absorption, contact angle, and microstructures. Investigating the mechanical properties of the PLA films is crucial due to the polymer's degradability over time. Polylactic acid films with different compounds containing PEG 400 and Tween 80 as plasticizers and ZnO nanoparticles were investigated for 14 months (in the first, second, third, fourth, and fourteenth months) in terms of mechanical properties. Finally, the obtained values were compared with standard values for packaging and their mechanical behavior was analyzed. Materials and Methods Experiments were performed in the post-harvest and central laboratories of Ferdowsi University of Mashhad, Iran. The films were prepared using the solvent casting method. First, PLA granules were dried for 24 hours at 60 degrees C and then 1 g of PLA in 50 ml of dichloromethane was dissolved at room temperature by magnetic stirring for 12 hours. ZnO nanoparticles, PEG 400 and Tween 80 were incorporated into PLA and DCM solution, 1 wt% PLA, 20 wt% PLA, and 0.25 wt% solution, respectively. To prepare films containing nanoparticles, nanoparticles and dichloromethane were sonicated with an ultrasonic probe for 10 minutes and then added to the base solution and stirred for one hour. Mechanical properties of the samples were determined based on the ASTM D882-02 standard method. A texture analyzer (H5 KS, Manchester, U.K.) was used for this test. Light absorption was studied using a spectrophotometer (CAMSPECM550, UK). The contact angle of the samples was measured using a goniometer (model 200-00, Ram & eacute;-Hart Instrument Co, Succasunna, USA) in accordance with the ASTM D5946-04 standard. The surface morphology of the samples was visualized using scanning electron microscope (LMU TESCAN BRNO-Mira3, Czech Republic). The results were analyzed using Minitab software version 18 (Minitab Inc, USA) and the graphs were created in Microsoft Excel 2013. Results and Discussion The neat PLA film has a smooth surface, and with the addition of nanoparticles or plasticizers, the surfaces become uneven. The addition of nanoparticles and plasticizers caused more opacity of the film and better protection against ultraviolet rays. The presence of plasticizers, especially Tween 80, increased the hydrophilicity of the films. Packaging films should be flexible and have ductile behavior and the addition of plasticizers caused ductile behavior. However, Tween 80 was not able to create stable ductile behavior. The stress-strain diagram shows that most samples displayed ductile behavior over 14 months, except for the neat PLA film and the film containing Tween 80 and nanoparticles. The values of tensile strength, elastic modulus, and elongation at break for low density polyethylene have been reported as 11.7 MPa, 260.4 MPa, and 225%, respectively. The lowest value of tensile strength (18.56 MPa) and elastic modulus (1114.68 MPa) were related to P400/T80 film. This difference shows the acceptability of polylactic acid in the packaging industry. The elongation value is much lower than the standard, indicating the need to modify this parameter. Conclusion The research findings revealed a significant effect of film type on mechanical properties, as well as a remarkable impact of storage time on tensile strength and elongation at break. The effect of various factors such as changes in the texture of the film due to the presence of plasticizers or non-uniform distribution of nanoparticles makes it impossible to determine a consistent trend for the effect of time on the films. The elongation at break for the produced films was much lower than the standard, which still needs to be modified due to the importance and sensitivity of this parameter in packaging. Polylactic acid has high tensile strength and high elastic modulus. Therefore, it can be combined with other polymers, various plasticizers, or nanoparticles at higher percentage to improve flexibility. The presence of plasticizers and nanoparticles in the film substrate increased opacity and enhanced protection against ultraviolet rays. The produced films were more hydrophilic compared to low density polyethylene.